This invention relates in general to a novel process for the preparation of non-chemically inhibited starch, and the products thereof.
A starch product is said to be "inhibited" when the granule is in some way strengthened so that in cooking some or all of the following is found:
1. The maximum swelling of the starch granule is lower. The hot viscosity is usually lower for moderate inhibition, but is somewhat higher for very low inhibitions.
2. If the starch cook is normally cohesive or phlegmy, this is reduced or eliminated.
3. If the original cook is clear, the inhibited product tends to be cloudy.
4. If the viscosity of the cook is measured as a function of time, e.g., by a Brabender Viscograph, the time of the peak viscosity is delayed or the peak may be entirely eliminated. The rate of increase in viscosity before the peak and the rate of decrease after the peak will both be lower for the inhibited starch.
These properties will be further discussed below.
In general, the inhibiting treatment increases the resistance of starch pastes to the thinning effects of prolonged agitation, heating or exposure to acids or alkalies. Thus, the sharp rise to a peak viscosity followed by a rapid drop in viscosity given by some starches (e.g., potato, tapioca) can be eliminated by inhibiting the starch. This property is of interest in thickeners for canned foods where loss of viscosity is undesirable. It is also of importance where cooked starches are subjected to prolonged pumping and heating operations in processing where a uniform viscosity must be maintained.
A slightly inhibited starch has a slower rate of gelatinization making the viscosity increase to a maximum more gradual, the extent of this change being related to the degree of inhibition. This property is useful where a delayed thickening on heating is desired, as in the retorting of canned foods. Additionally, a slight degree of inhibition is particularly valuable in reducing the rubbery, cohesive character of tapioca and potato starch pastes to a smooth, creamy texture. These products, prepared with bifunctional reagents acceptable for modifying food starches, are useful in pie fillings, salad dressings and puddings.
As the degree of inhibition of ungelatinized starch granules is increased, the rate of swelling is lowered. This can be carried to a point (about one crosslink in 20 glucose units) where the inhibited starch granules is completely resistant to normal cooking temperatures and can even be sterilized by auto-claving without becoming gelatinous. A product of this type is useful as a surgical dusting powder, particularly since any residual starch left in an incision is absorbed.
The normal way of inhibiting starch is to crosslink the starch granules. To this end starch granules are usually chemically treated at a high alkaline pH with bifunctional reactants that react with the hydroxy groups of two or more starch molecules, thereby resulting in a strengthened granule. The bifunctional reagents typically utilized in these reactions include epichlorohydrin, linear dicarboxylic acid anhydrides, organic dihalides, divinyl sulfone, phosphorous oxychloride and soluble metaphosphates. Formaldehyde also inhibits starch, but only under acid conditions. Chemical inhibition is ultimately accomplished by forming reinforcing links between molecules in the granules, thereby adding to the hydrogen bonding forces which hold the granules together. In these known methods, the bifunctional reagent is incorporated into the inhibited starch product.
Another way of inhibiting starch is the heat-moisture treatment of starch as described by L. Sair in "Methods In Carbohydrate Chemistry", Vol. IV (Starch), R. L. Whistler, Ed., Academic Press, N. Y. (1964) p. 283-285, in an article entitled "Heat-Moisture Treatment of Starches". This non-chemical method of producing inhibited starch granules is especially directed towards root starches. This method, however, requires use of high pressure equipment and according to one variation of the method the moisture contents of the starches are adjusted to from 18 to 27%.
It is an object of this invention to provide a novel and commercially practicable process of preparing an inhibited starch. It is a further object to provide such a process which inhibits the starch without introducing chemical additives and thus permit greater use of the inhibited starch in foods and pharmaceuticals. Various related objects and advantages of this invention will become apparent from the following description thereof.